Portal:Radiation astronomy/X-ray astronomy article/12

This is a composite image (X-ray - red, optical - blue & white) of the spiral galaxy M74 with an ultraluminous X-ray source (ULX) indicated inside the box. Image is 9 arcmin per side at RA 01h 36m 41.70s Dec +15° 46' 59.0" in Pisces. Observation dates: June 19, 2001; October 19, 2001. Aka: NGC 628, ULX: CXOU J013651.1+154547. Credit: X-ray; J. Liu (U.Mich.) et al., CXC, NASA - Optical; Todd Boroson/NOAO/AURA/NSF.

Ultraluminous X-ray sources (ULXs) are pointlike, nonnuclear X-ray sources with luminosities above the Eddington limit of 3 × 10³⁹ ergs s⁻¹ for a 20 M_☉ black hole.

Many ULXs show strong variability and may be black hole binaries. To fall into the class of intermediate-mass black holes (IMBHs), their luminosities, thermal disk emissions, variation timescales, and surrounding emission-line nebulae must suggest this. However, when the emission is beamed or exceeds the Eddington limit, the ULX may be a stellar-mass black hole. The nearby spiral galaxy NGC 1313 has two compact ULXs, X-1 and X-2. For X-1 the X-ray luminosity increases to a maximum of 3 × 10⁴⁰ ergs s⁻¹, exceeding the Eddington limit, and enters a steep power-law state at high luminosities more indicative of a stellar-mass black hole, whereas X-2 has the opposite behavior and appears to be in the hard X-ray state of an IMBH.